I sure wouldn't want to lose my ability to experience regardless of whether others can ever notice (or whether it's possible).

But according to the though experiment you'd set up, you wouldn't notice:

even to the point where if we ask her [the sleepwalker] "Are you awake?" she answers "Yes."

Granted, you go on to say,

... then it might be necessarily impossible for us outsiders to distinguish her sleep from her waking. But we feel an intuitive believe that she should be able to do so easily.

But that to me seems like a contradiction. We asked the sleepwalker if she was awake, and she said "Yes", after all. If she could in fact determine that she was sleeping, she'd say "No". Which brings me to your next point:

Your objection here strikes me a little bit like behaviorism. ...After all, thoughts didn't demonstrate themselves in behavior beyond being talked about, in which case it was the talk that was part of the scientific domain, not the thinking. The thing is, I know I think, and my strong impression is that others talk about "thoughts" for the same reason I do: they think.

As far as I understand, a hardcore behaviorist would actually claim that humans have no internal state and are basic reflex agents; that's obviously silly, so that is not my position. Instead, I actually agree almost completely with everything you'd said above.

You think thoughts, and your thoughts affect your actions. Unlike a simple reflex agent, you are able to actually think about your thoughts; this ability affects your actions even further. For example, when someone asks you, "are you awake ?", you can think about it for a moment, and say "Yes, probably", or "Most likely, not". You can also hold a lively discourse on the subject of your own thoughts. Thus, your actions -- including your speech -- are, in fact, evidence for your consciousness. A perfect sleepwalker, then, would have to perfectly emulate being conscious, as well; and I'm willing to stick my neck out and say that a perfect emulation of X is, in fact, X.

You say that you are "under the strong impression that others experience the world similarly" to yourself -- well, why is that ? I would argue that your "strong impression" is actually based on evidence (well, that, and possibly some biologically programmed response, but mostly evidence). You converse with others and they respond in certain ways that are consistent with the hypothesis that they, like yourself, are conscious. Sure, they could be perfect sleepwalkers, but that is a less parsimonious hypothesis.

Thus, again, I see no need for dualistic assumptions of any kind, which includes qualia. Besides, dualism doesn't actually explain anything, it just replaces one mystery with another, because now you have to explain how it is that qualia do what they do -- in the total absence of any empirical evidence, as well as any possibility of acquiring such. Well, ok, you don't have to explain it (I'm not the boss of you), but then you're stuck with a kind of mental elan vital that gets you nowhere.

P.S.: For what it's worth (which admittedly isn't much), I personally recall having both kinds of dreams: the kind where I knew I was dreaming, and the kind that seemed so real that, upon waking, it took me some time to figure out whether or not the events in the dream had actually occurred. Thus, I have no trouble imagining a perfect sleepwalker who would answer "No" when you asked her whether she was dreaming or not.

(edited for style)